In drop tests against the rear plane of the cartridge case in ammunition units of the above-disclosed type, it has proved that the shell shows a tendency to slide into the cartridge case. Neither the crimp-and-groove union nor the bourrelet of the shell is capable of resisting the shell. The risk of collision between a tracer cartridge mounted on the rear plane of the shell and the primer is obvious. Such a collision will result in ignition of the tracer cartridge, which may have serious consequences in connection with ammunition handling.